Generally, semiconductor memory devices are the most essential microelectronic elements for digital logic designs such as computers and applications based on microprocessors pertaining to a range from satellites to consumer electronic technologies. Therefore, the advancements of technologies of fabricating semiconductor memories, including process improvement and technology development obtained through scaling for higher integration and speed, contribute to establishment of performance standards of other digital logics.
Semiconductor memory devices are generally classified into volatile memory devices and nonvolatile memory devices. In the volatile memory devices, logic information is stored by establishing the logic state of a flip-flop in case of a static random access memory and charging a capacitor in case of a dynamic random access memory. In the volatile memory devices, data is stored and read while power is supplied, whereas data is lost when power is shut off.
In the nonvolatile memory devices such as MROM, PROM, EPROM, EEPROM, and PRAM, data is retained even when power is shut off. The data storage state of the nonvolatile memory devices is permanent or reprogrammable according to applied fabrication technologies. The nonvolatile memory devices are used to store programs and microcodes in various applications such as computers, avionics, communications, and consumer electronic technologies. A combination of volatile and nonvolatile memory storage modes in a single chip may also be used in devices such as nonvolatile RAMs (nvRAMs) in a system that requires quick and reprogrammable nonvolatile memories. In addition, specific memory structures including some additional logic circuits have been developed to optimize the performance for application-oriented tasks.
In the nonvolatile semiconductor memory devices, it is not easy for general users to renew stored contents because MROM, PROM, and EPROM are not free to erase and write. However, since nonvolatile semiconductor memory devices such as EEPROM and PRAM are possible to electrically erase and write, their applications are being expanded to auxiliary memory units or system programming that requires continuous renewal.